Breastfeeding of human infants has been shown to provide protection against many pathogenic and pathologic conditions, and much of that protection is mediated by naturally occurring glycans in milk. We have recently identified a new glycan, hyaluronan (HA), in human milk, which occurs at the highest concentration during the early stage of human lactation and decreases gradually over time. Human milk HA levels are markedly higher than in commercial infant formulas. Importantly, we have also discovered that feeding purified HA of a specific size range to mice is protective in an epithelial damage/ bacteria-induced colitis model. Additionally our data indicate size-specific, HA fragment driven anti-microbial responses by intestinal epithelium. A special attribute of HA, different from other milk glycans, is its function as an endogenous ligand of the bacterial recognition Toll-like receptors (TLRs) 4 and 2, which are known to be important sensors that trigger the innate host defense system. Thus HA may be a way to stimulate the uncolonized infant gut to protect against pathogens. Therefore we hypothesize that HA in milk, in a size dependent manner, provides innate host protection against pathogens in the gut of immune-naive infants and children. Preliminary studies from our lab show HA size-specific upregulation of antimicrobial responses in intestinal epithelial cells in cell cultures and mouse models. Among the virus response proteins, our data show IFIT1 is markedly induced by HA, specifically of the 35kDa size range, and a combination of antibodies to TLR4 and TLR2 completely blocks this effect. Importantly, the HA we can isolate from milk also has the same stimulatory ability. The HA 35kDa fragments also specifically induce human 2-defensin 2, one member of the antimicrobial peptide family that is important in excluding pathogens and maintaining proper intestinal bacterial colonization. Again, the HA isolated from human milk has the same ability. We envision that our proposed investigation could ultimately lead to identifying defined dietary HA supplements that would be therapeutic to infants with NEC, infants who are formula fed and children prone to gastrointestinal infectious disease. HA would be an attractive therapeutic because it is: 1) not toxic or immunogenic due to its simple unmodified carbohydrate structure; 2) easily administered; 3) already being produced commercially on a large scale as a health food supplement as well as for medical applications. Based on our preliminary data and the previously reported biological effects of HA, we think the key to its protective potency will depend on the HA polysaccharide size(s) needed to mediate individual host responses.